A SAW-transducer normally includes two comb like electrodes that have engaging electrode fingers. This structure is arranged on a piezoelectric substrate and is used for electro acoustic transformation of a (high-frequency) electric signal into an acoustic wave and vice versa. SAW-transducers are especially used in band pass filters of data transfer systems.
The electric behavior of a filter is normally defined by a transfer function (frequency response of the amplitude and the phase of the figurative signal). An important description of the transfer function especially is the waviness (ripple) of the group delay in the pass band of the filter.
The acoustic behavior of a transducer can be characterized (locally in the longitudinal direction) by weighting functions (exciting function as well as reflection function). The weighting functions depend on the longitudinal coordinate and describe the distribution of this excitation as well as reflection strength determined in the transducer. The weighting functions of a transducer can be determined on the basis of the preset electric filter qualities. From the calculated weighting function, one can infer the necessary connection sequence and design of the electrode fingers. However, this connection is not clear.
What are familiar are, for example, what is known as SPUDT-filters (SPUDT=single phase unidirectional transducers) that are distinguished by the fact that its cells—SPUDT-cells—serve to direct the irradiation of the acoustic waves in each case. The directional irradiation comes about through the constructive overlay of the excited and reflected waves in one direction and/or destructive overlay of the excited and reflected waves in the opposite direction.
With the filter design, a low insertion loss of the electric signal in the pass band and at the same time a high suppression of the signal in the cutoff sections of the filter are striven for. In order to be able to simultaneously guarantee a low insertion loss of the signal in the pass band and a high suppression of the signal in the filter attenuation bands that are specified near the pass band, a high flank steepness of the transfer function of the filter is striven for. In order to increase the edge steepness of the transfer function, the impulse response of the filter in the time domain must be extended. For this, in SPUDT-filters for example, the cell sequence is selected in such a way that it comes to a multiple reflection of the acoustic waves in the transducer, whereby the circuit of the wave is accordingly extended in the transducer. The excited wave and the (multiply) reflected wave components overlap constructively or destructively according to their relative phases. The increase of reflection strength that is necessary for this has the disadvantage that thereby also the waviness in the frequency characteristic of the group delay also increases.
A low insertion loss in the pass band with a high suppression in the filter attenuation bands can, in principle, be reached in a two track SPUDT-filter. The passage from a constructive overlay to a destructive overlay of wave components in ranges of bandwidth and cutoff regions that densely arranged together by frequency is, however, impeded in both tracks. This passage defines the edge steepness of the transfer function of the filter. What is especially unsuccessful is achieving a low waviness of, for example, below 50 ns in the frequency characteristic of the group delay in the pass band of the filter with a high edge steepness of the transfer function.
The connection between the filter geometry applicable for propagation of the acoustic waves and the electric filter qualities is extremely complex and can only be assessed for the filter geometry in question with help of costly computational models that consider all multiply reflected wave components. Therefore one cannot simply deduce how a filter should be constructed with electric properties in question.
In addition, it is known that the weighting function of a filter can be improved through an appropriate optimization, so that the non-linearity of the phase response is reduced to a limited extent. However, this is interconnected in the transducer with an undesirable diminution of the reflection strength and with respect to the attainable waviness of the group delay in the pass band leads to insufficient results.
The task of the present invention is to indicate a transducer that is employable in a SAW-filter and that thereby serves to reach a low waviness in the frequency description of the group delay in the pass band of this filter with a high suppression in the filter attenuation band without reduction of the reflection strength.
This task is solved in accordance with the invention by an electro acoustic transducer with the characteristics of claim 1 or 2. Advantageous designs of the invention are to be taken from further claims.
The idea that underlies the first implementation form of the invention is that the resulting phase response of the filter can be influenced out of a complicated overlay of multiply reflected wave components by having uniformly formed cells in a longitudinal direction (the direction of wave propagation) scaled to each other, that is, compressed or expanded.
The acoustic length of a wave that crosses a cell in a longitudinal direction is established by the length of the cell. In passing through the cell, the acoustic wave, which is excited by a specific frequency, e.g., the center frequency of the filter, finds a phase shift corresponding to the cell length. By altering the acoustic length, e.g., by compressing or expanding the cell in a longitudinal direction, the phase shift of the wave component in this cell also changes. By a suitable scaling of the cells—or alternatively by extending the displacement of the last electrode finger of a cell and the first electrode finger of the subsequent cell while retaining the scaling of the finger ordering of the corresponding cell—the non-linearity of the phase response, which comes from multiple reflections and which represents an integral characteristic of the filter, can be evened out. It is thereby possible to reach an especially low waviness of the frequency characteristic of the group delay in the pass band of the filter of, for example, less than 50 ns.
In its initial implementation, the invention provides a transducer that operates by surface waves for electro acoustic filters, especially band-pass filters, that shows at least one acoustic track in which an acoustic surface wave can be excited at the center frequency of the transducer.
The acoustic track is apportioned in a longitudinal direction in cells. At least some of the cells are constructed as function cells (SPUDT cells) that serve to excite and/or reflect the acoustic wave, whereby at least two function cells of a certain cell type are provided.
A cell type is defined by: 1) the connection sequence of the fingers, 2) the relative width of the finger with reference to the wavelength and 3) the relative displacement between the fingers with respect to the wavelength.
The absolute cell length of a function cell is, for example, defined by the displacement of the left edge of the first electrode finger of the function cell that is assigned to the corresponding cell type up to the corresponding edge of the first electrode finger of the subsequent cell, whereby the acoustic wave excited at center frequency in passing through the acoustic distance, which is equal to the absolute cell length, essentially learns a phase shift of 2π or a whole number multiple of 2π.
It thereby holds for at least two, especially exciting cells of the same cell type that the acoustic wave that is excited with the center frequency of the filter in the transducer, suffers phase shifts φ and φ′ in the respective differing function cells in traversing an acoustic distance from the beginning of the function cell of the corresponding cell type up to the beginning of the subsequent cell.
The absolute cell lengths of all function cells in a known transducer are selected in such a way that the wave with a certain frequency (for example in the center frequency) always suffers a phase shift of exactly 2π in traversing these cells.
However, the wave corresponding to the center frequency that meets with a function cell with an acoustic distance modified in accordance with the invention, suffers a phase shift deviating by the value of 2π in traversing this function cell. Therefore, after traversing the modified function cell, this wave does not match any more the following cells exactly in phase. The contributions of individual function cells to the resulting wave are easily phase-postponed vs. each other to a small extent, which allows an adaptation of the phase of the resulting wave.
A transducer according to the invention has the advantage that a phase adaptation of the acoustic wave in the pass band of the corresponding filter without diminution of the reflection strength is achieved and consequently without diminution of the flank steepness of the transfer function. A low waviness of the group delay can thereby be reached in this transducer with a suitable optimization of the cell lengths.
In the preferred variation of the first implementation form of the invention, at least two of the function cells of a specific cell type are provided that show a geometrical similarity in a longitudinal direction up to the scaling, whereby they are scaled differently in this direction vs., each other, so that in each case they represent different circuits vs. each other for the acoustic wave with a certain frequency.
The difference in the scaling of the differently scaled function cells of the same cell type preferably amounts to between 0.1% and 20%.
In another advantageous variation of the invention, it is proposed, in at least two function cells of the same cell type that show identically constructed electrode finger groups the differently select the displacement between the end position electrode finger of the corresponding function cell and the electrode finger dedicated to this electrode finger of the subsequent cell in different function cells of the same cell type. In this text, an electrode finger group is understood as all electrode fingers of the corresponding function cell. The identically constructed electrode finger groups display in particular a same connection sequence of the electrode fingers the same (with respect to wave length) finger width of the corresponding electrode finger and those in both groups are identical to each other (with respect to wave length) in displacements between the electrode fingers.
The width of the electrode finger is preferably selected in such a way that it essentially amounts to mλ/16 or deviates from this value by a maximum of 20%, in which m is an integer.
In another variation of the invention, at least one further acoustic track is anticipated that shows essentially the same characteristics as the first named acoustic track, whereby the acoustic tracks are arranged parallel to each other and electrically interconnected with each other.
The second implementation form of the invention relates to a transducer with an acoustic track that is apportioned in a longitudinal direction in cells of different cell types, whereby at least two function cells of the same cell type present at least one wide and at least one narrow electrode finger. In the different function cells of the same cell type, as according to the invention, different reflection strengths are attained, in that cells of the same cell type are optimized as follows.
In the function cells that are to be optimized, each according to whether the reflection strength should be reduced or raised, a part of the finger width of the narrow, that is the non-reflective, finger is cut off and this part of the finger width is added to the wide, that is reflecting, fingers, or a part of the finger width of the wide finger is cut off and this part of the finger width is added to the narrow fingers. In the cells that optimized as according to the invention, the sum of the relative changes with respect to the cell length of all electrode fingers is zero.
By a narrow electrode finger in the sense of the discovery is understood an electrode finger whose width in comparison to the width of an electrode finger that is designated as wide electrode finger is, for example, smaller by a factor of at least 2.
The optimized cell differs from the non-optimized by the fact that the sum of the relative widths of all narrow electrode fingers with respect to cell length is variously selected by a difference of +α(α≠0, preferably /α/<0.1). At the same time, in the optimized cell, the sum of the relative widths with respect to cell length of all wide electrode fingers is variously selected by a difference of −α, so that the relative latitudinal alteration of all electrode fingers is zero. The displacements for the fingers in the different cells of the same cell type preferably remain constant.
Several cells of the same cell type differ from each other by the fact that at most 20% of the width of one or more fingers is distributed on another or several other fingers.
The alteration of the relative finger widths according to the invention in the transducer is defined through a weighting function (the reflection), which defined by only a few parameters.
The second implementation form of the invention has the advantage that the reflection strength of individual cells can be continuously varied with justifiable expenditure of the CPU time.
Which fingers are narrowed in the optimization of the reflection strength according to the invention and which are widened depends on the cell class (e.g. EWC-cell class, Hanma-Hunsigner cell class) and the cell type. The size, by which the fingers are widened and/or narrowed can be easily decided from a weighting function defined by a few parameters.
By the optimization of the reflection strength of individual cells as accorded by the invention there was particular success with equalizing the discretization errors in the realization of continuous weighting functions of reflection into a discreet transducer geometry—with a limited number of the cells in the acoustic track. This is important especially in filters with a low center frequency and a large wave length resulting from it, since only a few cells can be accommodated on the length that is available, whereby the scanning of the continuous weighting functions fails very extensively with the same large discretization errors by analogy.
The first and the second implementation form of the invention can be combined together whereby a function cell optimized according to the second implementation form is, for example, additionally compressed vs. the length of a non-optimized cell, or lengthened. Thereby the metallization ratio and consequently also the spread speed of the acoustic wave is the same in an optimized and a non-optimized cell.
Through the additional scaling of the cells changed in accordance with the second implementation form of the invention, there was particular success in compensating for the unintentional offset of the reflection and exciting centers that come into existence from the finger latitudinal alteration.
The invention becomes in the following the invention is explained in more detail in the following on the basis of implementation examples and their related figures. The figures show various implementation examples of the invention by means of schematic presentations that are not to scale. Identical or similarly functioning parts are marked with same reference signs. Show in schematic top views are: